In a typical video encoder implementation, digital video frames are segmented into video slices at the encoder side to reduce the impact of data losses during transmission. Each video slice is independently encoded and decoded. When an error is detected, a decoder conceals all macroblocks (MBs) within the corrupted slice. Concealment prevents the presentation and propagation of wrongly decoded MBs that could otherwise degrade the quality of the video presented to the user.
A video slice typically starts with a resynchronization marker (RM) that can be used by the decoder to re-establish synchronization when errors are detected. To minimize data loss, a video slice ideally corresponds to the smallest loss unit within a video transmission system. The smallest loss unit is the smallest unit of information transmitted across the communication channel. In a wireless system, the smallest loss unit is referred to as a logical transmission unit (LTU), which may have a fixed or variable length.
If each video slice is the size of an LTU, any errors due to LTU loss will be localized within the lost slice without affecting other slices. However, it is difficult to achieve exact alignment between video slices and LTUs. In particular, RMs can only be placed at MB boundaries, and cannot be placed arbitrarily within the video frame. As a result, it is difficult for the encoder to adjust the slice size to fit exactly within a given LTU.
One technique for LTU-slice alignment involves slice-level rate control to adjust the quantization step-size of MBs to adjust slice length. Unfortunately, this technique adds significant complexity to the encoder design, and is not necessarily exact. Another technique involves encoding a slice until its size exceeds a predefined length, and then adding padding bytes between the end of the encoded video and the end of the LTU. However, this approach undermines bandwidth efficiency.